Water-based liquid ink compositions have conventionally been widely used for ink-jet printing. However, the recording paper media to which such water-based ink compositions are applicable are limited to converted papers because the ink compositions are apt to permeate plain paper to cause "coalescence". For use in recording on overhead projector (OHP) sheets, the water-based ink compositions necessitate a special treatment of the sheet surface because of the poor drying properties of the inks. Under these circumstances, a hot-melt ink-jet recording technique has been proposed in which a hot-melt ink composition based on a wax or the like which is solid at room temperature is used as an ink composition which provides a satisfactory print quality irrespective of paper quality. In this technique, the ink composition is liquefied by heating or another means, and the melt is ejected by applying some energy thereto. The ejected ink droplets are adhered to a recording medium and allowed to cool and solidify to thereby form ink dots.
Major advantages of the above ink-jet recording technique are that the ink does not cause fouling during handling because it is solid at room temperature, and that it does not cause nozzle clogging because ink evaporation, which occurs when the ink is in a molten state, can be minimized. Still another advantage thereof is that the ink is free from "coalescence" because it solidifies immediately after adhesion to the recording medium. Consequently, various recording media ranging from Japanese paper to drawing paper and post card paper can be used without any pretreatment or the like. In U.S. Pat. Nos. 4,390,369, 4,484,948, 5,350,789, and 5,703,145 are described ink compositions which provide a satisfactory print quality irrespective of paper quality. JP-A-5-311101 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") describes an ink-jet hot-melt ink composition having excellent light transmission which contains a polyamide and a fatty acid amide. Other ink-jet hot-melt ink compositions having excellent light transmission are described in JP-A-5-194897 and JP-A-6-107987, which contain a glyceride.
On the other hand, a generally employed technique for improving the weather resistance of prints is to use pigments as colorants for inks. Such pigmented inks are used in various printers including laser printers, melt transfer printers, liquid-ink-jet printers, and solid-ink-jet printers. For example, many reports have been made on pigmented inks for use in solid-ink-jet printers, e.g., in JP-A-3-37278, JP-A-4-339871, JP-A-5-16343, JP-A-5-105832, JP-A-6-49400, JP-A-6-228479, JP-A-6-228480, JP-A-6-306319, JP-A-7-109432, JP-A-7-196968, JP-A-7-278477, JP-A-7-306319, JP-A-7-316479, JP-A-7-331141, and JP-A-8-295836. In JP-A-61-159470 is described a hot-melt ink comprising a vehicle comprising an alcohol having from 18 to 24 carbon atoms and, contained in the vehicle, graphite particles which have been dispersed beforehand in an oil carrier compatible with the vehicle.
However, the hot-melt ink compositions described above have a drawback that it is generally difficult to maintain compatibility between the components and, hence, the ink compositions in a molten state are apt to suffer colorant separation especially when the colorant used is an organic pigment having excellent weather resistance. As is well known, the sedimentation of particles dispersed in a liquid depends on the diameter of the particles, the viscosity of the dispersion medium, and sedimentation period. The higher the viscosity of the dispersion medium, the less the particles sediment. On the other hand, from the standpoint of using an ink in printing with an ink-jet printer, lower ink viscosities are advantageous for higher printing speeds and higher densities and are suitable for highly reliable printing. Thus, the prevention of pigment sedimentation in an ink and the performance of the ink in printing have been inconsistent with each other.
As described above, the hot-melt ink-jet recording technique in which an organic pigment having excellent weather resistance is used as a colorant has many advantages over the ink-jet recording technique in which a dye is used. The former technique is hence expected to be used not only in OA apparatuses, domestic printers, facsimile telegraphs, and the like but in applications such as outdoor or indoor posters, large signboards, the decoration of motor vehicles or elevators, and fabric printing. However, the above-described problem that the prevention of ink separation is inconsistent with highly reliable print quality has been an obstacle to the commercial use of the technique. There has frequently been such a trade-off that pigments having relatively good dispersibility in vehicle resins generally have poor weather resistance.